Method for producing flame- and smoke-resistant polyurethane foam sheet by incorporation of a urea derivative in reaction mixture

ABSTRACT

The flexible polyurethane foams having high flame resistance and smoke resistance can be produced by reacting a polyhydroxyl compound and a polyisocyanate with a urea derivative selected from the group consisting of 
     (a) a compound represented by the general formula ##STR1##  wherein at least one of R 1 , R 2 , R 3  and R 4  represents hydroxylalkyl, alkoxyalkyl, alkyl and aryl groups and the remainder represents hydrogen atom, and 
     (b) a urea-formaldehyde condensation product in the presence of a catalyst, a surfactant and a blowing agent. 
     These polyurethane foams can easily provide the favorable foam sheets by compression-molding these foams under heating.

This is a Division of application Ser. No. 778,282, filed Mar. 16, 1977,now U.S. Pat. No. 4,180,631 issued Dec. 25, 1979.

BACKGROUND OF THE INVENTION

1. Field of the Invention:

The present invention relates to flexible polyurethane foams having ahigh flame resistance and smoke resistance and to heatcompression-moldings thereof.

2. Description of the Prior Art:

Polyurethane foams have a broad use from seat and cushion materials forfurniture, vehicles and ships to cloths and architecture materials.Furthermore, it has been known that foam sheets can be obtained bycompression-molding the polyurethane foam under heating by utilizing thethermoplasticity of flexible polyurethane foams and the foam sheets havebeen broadly used for both surface adhesive tapes; imitative leathers;packing cushion materials for protecting optical machines, precisionmachines and glass; a slip preventer for a mat; prevention of shocknoise when a door is opened and closed; wrapping and the like.

Substantially all the above described materials are inflammable and inorder to satisfactorily adapt the foam sheet to the above describeduses, it has been recently demanded to use flame resistant polyurethanefoams.

However, the well known processes, that is, use of phosphorus compounds,a combined use of halogen and phosphorus compounds, use of a metaloxide, such as antimony oxide, use of a reactive flame retardant and thelike can give a certain degree of flame resistance to polyurethane foamsbut the generation of smoke when burning is rather larger than the casewhere no flame retardant is used and there has been a great problem inview of the smoke emission.

Moreover, the process heretofore carried out for molding the foam into asheet comprises compressing a flexible polyurethane foam at a hightemperature 150°-200° C.for a long time and this process has not beenpreferable in view of save of energy and productivity.

SUMMARY OF THE INVENTION

An object of the present invention is to provide flexible polyurethanefoams having a high flame resistance and smoke resistance.

Another object of the present invention is to provide polyurethane foamsheets having a high flame resistance and smoke resistance.

The other object of the present invention is to provide a method forproducing said polyurethane foam sheet.

One aspect of the present invention is production of flexiblepolyurethane foams having a high flame resistance and smoke resistanceby reacting a polyhydroxyl compound and polyisocyanate with a ureaderivative selected from the group consisting of ##STR2## wherein atleast one of R₁, R₂, R₃ and R₄ represents hydroxyl group, alkoxyalkylgroup, alkyl group or aryl group and the remainder represents hydrogenand

(b) urea-formaldehyde condensation product, in the presence of acatalyst, a surfactant and a blowing agent.

Another aspect of the present invention is production of polyurethanefoam sheets having a high flame resistance and smoke resistance byreacting a polyhydroxyl compound and polyisocyanate with a ureaderivative selected from the group consisting of ##STR3## wherein atleast one of R₁, R₂, R₃ and R₄ represents hydroxyl group, alkoxyalkylgroup, alkyl group or aryl group and the remainder represents hydrogen(b) urea-formaldehyde condensation product, in the presence of acatalyst, a surfactant and a blowing agent to form a foam and thencompressing the resulting foam under heating. Hitherto, it has beenknown that a flame resistant polyurethane foam is produced by using ureabut this process has the following various drawbacks and the resultingpolyurethane foams have not been fully satisfied in view of the physicalproperties and the flame resistance. That is, (1) the foam is high indensity, (2) the compressive permanent stress increases and (3) theflame resistance lowers when the foam is left to stand under moistureand as time passes.

According to the present invention, polyurethane foams having a highflame resistance can be produced by using an inexpensive urea derivativewithout using any flame retardant, so that the present invention iseconomic.

Furthermore, the polyurethane foams according to the present inventionare not only excellent in flame resistance but also that smoke emissionis restrained, so that the foams are further preferable in view ofsafety upon burning.

In addition, according to the present invention, the polyurethane foamsheets can be easily obtained by compressing the polyurethane foams at atemperature of 80°-120° C., so that the foam sheets are preferable inview of saving of energy and productivity. The production of thecompression-moldings from usual polyurethane foams under such conditionsis difficult.

Accordingly, the polyurethane foams obtained according to the presentinvention are useful for various cushion materials, interior materials,heat insulation materials, and the polyurethane foam sheets are usefulfor both surface adhesive tapes, imitative leathers, packing, cushionmaterials and the like.

BRIEF EXPLANATION OF DRAWINGS

FIG. 1 is a photograph showing the shapes of polyurethane foam accordingto the present invention (left side) and polyurethane foam which is notin accordance with the present invention (right side) prior tocompression under heating and

FIG. 2 is a photograph showing the shapes of the polyurethane foamsshown in FIG. 1 after compressing under heating.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The polyhydroxyl compounds to be used in the present invention arepolyethers or polyesters having a terminal hydroxyl group which areusually referred to as polyols. The polyether polyols are usuallyobtained by ring opening addition polymerization of alkylene oxides,such as ethylene oxide, propylene oxide and the like, with initiatorscontaining active hydrogen atoms, such as glycerine, trimethylolpropane,pentaerythritol, sorbitol, saccharose and the like, in the presence ofan alkali catalyst. The polyester polyols are usually obtained by thepolycondensation reaction of dicarboxylic acids, such as adipic acid,sebasic acid, phthalic acid, isophthalic acid, terephthalic acid and thelike, with initiators containing active hydrogen, such as ethyleneglycol, propylene glycol, diethylene glycol, glycerine, trimethylolethane, trimethylol propane and the like.

In the present invention, the polyhydroxyl compounds having a numberaverage molecular weight of 3,000-6,000 and containing at most 70 mole %of primary hydroxyl group in its molecule terminal are generally used.

Among the polyhydroxyl compounds as described above, there may bepreferably used polyether polyols. Especially, poly(oxypropylene)triolobtained by reacting glycerine with propylene oxide is advantageouslyused.

As the polyisocyanates to be used in the present invention, use may begenerally made of tolylene diisocyanate. Particularly, tolylenediisocyanate, wherein 2,4- and 2,6-isomers of isocyanate group are mixedin a mixture ratio of 80/20 or 65/35 (weight ratio), is preferable inview of a low cost and utility. Tolylene diisocyanate may be pureproduct, crude product or the mixture thereof. As the otherpolyisocyanates, use may be made of diphenyl diisocyanate,chlorophenyl-2,4-diisocyanate, p-phenylene diisocyanate, xylylenediisocyanate, polymethylene-polyphenyl isocyanate and the like, or themixtures of these polyisocyanates with tolylene diisocyanate.

The amount of polyisocyanate used based on the total amount of thepolyol and the other compound having active hydrogen atoms, that is, theisocyanate index (NCO index) is in the range of 80-130, but whenconsidering the balance of the flame resistance and the smoke resistanceto the general physical properties of the produced polyurethane foam,the isocyanate index is preferred to be within the range of 100-110.

The urea derivatives to be used in the present invention are selectedfrom the following compounds.

(a) Compounds represented by the general formula ##STR4## wherein atleast one of R₁, R₂, R₃ and R₄ represents a hydroxyalkyl-, alkoxyalkyl-,alkyl- or aryl group and the remainder represent hydrogen atoms; and

(b) Urea-formaldehyde condensation product.

As the compounds represented by the general formula (a), mention may bemade of hydroxylalkyl substituted urea compounds, such as N-methylolurea, N,N'-dimethylol urea, N,N,N'-trimethylol urea,N,N,N',N,'-tetramethylol urea, N-ethylol urea, N,N'-diethylol urea,N,N-di(β-hydroxyethyl) urea; alkoxyalkyl substituted urea compounds,such as N,N'-dimethoxymethyl urea, N,N'-dimethoxyethyl urea,N,N'-diethoxymethyl urea, N,N'-diethoxyethyl urea; alkyl substitutedurea compounds, such as N-methyl urea, N,N'-dimethyl urea, N-ethyl urea,N,N'-diethyl urea; and aryl substituted urea compounds, such as N-phenylurea, N,N'-diphenyl urea.

As the urea-formaldehyde condensation products (b), the compoundprepared by reacting one mole of urea with 1-4 moles of formadehyde inthe presence of an alkali catalyst, and commercially available variousurea-formaldehyde condensation products may be used.

In the present invention, as the urea derivative, there is preferablyused N,N'-dimethylol urea, N,N'-dimethoxymethyl urea, N,N'-dimethyl ureaand a urea-formaldehyde condensation product. Particularly,N,N'-dimethylol urea is preferred for production of polyurethane foamsheets having an excellent flame resistance.

The amount of the urea derivative to be used in the present inventionmust be within the range of 2-50 parts by weight based on 100 parts byweight of the polyhydroxyl compounds. The use of less than 2 parts byweight of the urea derivative can not provide a sufficient flameresistance to the resultant polyurethane foam, and when the amount ofthe urea derivative is more than 50 parts by weight, a favorable foamcan not be obtained. Especially, the amount of said urea derivative tobe used is the most favorable in the range of 5 to 20 parts by weight byconsidering the balance of the foaming reaction and the process ofcompression-molding.

In the present invention, as a curing catalyst for said urea derivative,use may be made of ammonium hydrogen-phosphate, ammonium nitrate,ammonium formate, hydrogen chloride salts of alkyl amines (alkyl meansmethyl, ethyl, propyl and the like), magnesium chloride and the like.The amount of said catalyst is within the range of 0.5-3 parts by weightbased on 100 parts by weight of the polyhydroxyl compound.

As the catalyst to be used in the present invention, use may be made ofalready known catalysts which have been commonly used in this field. Thecatalysts include organometallic compounds, such as stannous octoate,dibutyltin dilaurate and the like; and amines, such astriethylenediamine, triethylamine, N-methylmorpholine,N-ethylmorpholine, tetramethylbutanediamine,pentamethyldiethylenetriamine, N,N-dimethylethanolamine,bis(β-dimethylaminoethyl) ether and the like. The above describedcatalysts can be used alone or in admixture. The amount of catalyst tobe used in the present invention is not particularly limited and can bevaried in a wide range. However, the catalyst is generally used in anamount of 0.01-5.0 parts by weight, preferably 0.01-2.0 parts by weight,based on 100 parts by weight of the polyhydroxyl compound used in theproduction of the polyurethane.

As the surfactant, use may be made of well known silicone surfactants,for example, polydialkylsiloxanes and polysiloxane-polyoxyalkylene blockcopolymers. The kind and use amount of the silicone surfactant are notlimited as far as the object of the present invention is attained, butit is preferable to use polysiloxane-polyoxyalkylene block copolymeralone or in admixture. The use amount of silicone surfactant is 1.0-5.0parts by weight, preferably 1.0-2.0 parts by weight, based on 100 partsby weight of the polyhydroxyl compound.

The blowing agent to be used in the present invention is water orvolatile liquid having a low boiling point. The volatile liquid having alow boiling point includes, for example, trichloromonofluoromethanedibromodifluoromethane, dichlorodifluoromethane,dichlorotetrafluoroethane, monochlorodifluoromethane,trifluoroethylbromide, dichloromethane and the like. These blowingagents may be used alone or in admixture.

The polyurethane foam obtained in the present invention is a flexiblepolyurethane foam having a density within the range of 20-50 kg/m³. Ifdesired, the density of the foam can be further controlled by addingwater, another blowing agent or a urea derivative or by selecting thekind of the polyhydroxyl compound or the polyisocyanate. Further, in thepresent invention, a crosslinking agent, a flame retardant, pigment andthe like may be added in addition to the above described components.

The production of the polyurethane foams according to the presentinvention can be carried out by conventionally known process. Forexample, an one shot process, wherein the polyhydroxyl compound, theurea derivative, water, the catalyst and the surfactant are concurrentlymixed together with the polyisocyanate to cause reaction and foaming, isusually adopted. The polyurethane foam obtained as described above inthe present invention has a high flame resistance and smoke resistance.

In the present invention, in order to produce a polyurethane foam sheet,it is necessary to compress the polyurethane foam obtained above underrestricted condition which are determined considering heatingtemperature, compression degree, compressive stress and heat-compressingtime.

The desired polyurethane foam sheet of the present invention, can beeasily produced by compressing the polyurethane foam obtained aboveunder the conditions of a pressure of 5-200 kg/cm² and a temperature of80°-120° C. for 2-10 minutes. For example, a polyurethane foam sheethaving a thickness of 2 mm is obtained by putting the polyurethane foamhaving a thickness of 20 mm obtained in the present invention into amold having a depth of 2 mm and by compressing said foam under theconditions of a temperature of 100° C. and a pressure of 100 kg/cm² for10 minutes. On the other hand, a general flexible polyurethane foam isnot at all compression-molded under such conditions.

The following examples are given for the purpose of illustration of thisinvention and are not intended as limitations thereof. In the examples,"parts" means by weight unless otherwise indicated.

In the examples, the burning test was carried out according to ASTMD-1692-59T. The smoke emission test was carried out in the followingmanner. One g of a sample foam is burnt in an electrical furnace havingan inner diameter of 100 mm and a depth of 250 mm by means of anelectrical heating wire of 3 KW while flowing air thereover at a rate of2.0 l/min, and the generated smoke is gathered in a box of 50×50×50 cm.The maximum value of the extinction coefficient per unit weight of thesample is measured by means of a photoelectric detector, and expressedas the smoke generation coefficient C_(Smax) (m⁻¹). The smoke resistanceof a foam was judged by the smoke generation coefficient. The smallerthe coefficient, the higher the smoke resistance is.

EXAMPLE 1

Ten parts of N,N'-dimethylol urea was previously dispersed in 100 partsof poly(oxypropylene) triol (made by SANYO Chemical Industries, Ltd.trademark. GP-3000; number average molecular weight: about 3000,hydroxyl value: 56) and 4.2 parts of water, 0.2 part of stannousoctoate, 0.1 part of triethylenediamine and 1.7 parts of siliconesurfactant (trademark: SH-190, made by Toray Silicone Co.) were addedthereto. The mixture was thoroughly mixed and stirred by means of apropeller stirrer for about 30 seconds. 53.2 parts of tolylenediisocyanate (2,4-isomer: 2,6-isomer=80:20) was added to the resultinghomogeneous mixture. When the obtained mixture was stirred at a highspeed, the reaction mixture reached a creamy state in about 18 seconds.Then, the reaction mixture was rapidly poured into a paper mold, foamingbegan and foam rising was completed after about 119 seconds andso-called healthy bubbles were observed.

The said polyurethane foam was put in an oven at 120° C. and after-curedfor 20 minutes, after which the physical properties were measured afterone week. The density of the foam was 23.5 kg/m³ and the foam had aself-extingushing property (burning extent: 6.1 cm, burning time: 18seconds) in the burning test of ASTM D-1692-59T. In the smoke emissiontest, C_(Smax) was 0.75 m⁻¹, while C_(Smax) of a commercially availablepolyurethane foam under the same measuring condition was 0.87 m⁻¹ andthe C_(Smax) of an ordinarily used flame resistant polyurethane foamhaving added thereto 25 parts of tris-(2,3-dichloropropyl) phosphate was1.03 m⁻¹. As seen from these data, the smoke resistance of thepolyurethane foam of the present invention was very much higher thanthat of the comparative polyurethane foams.

EXAMPLE 2

Using the same starting materials and operations as described in Example1, 20 parts of N,N'-dimethylol urea was added to obtain a foam.

The obtained flexible polyurethane foam had a self-extinguishingproperty (burning extent: 9.2 cm, burning time: 35 seconds) in theburning test of ASTM D-1692-59T. The smoke generation coefficientC_(Smax) was 0.66 m⁻¹ which was far lower than the (1.03 m⁻¹) of theordinarily used flame resistant polyurethane foam. The polyurethane foamof the present invention is thus seen to have high smoke resistance.

EXAMPLE 3

Using the same starting materials and operations as described in Example1, foaming was effected by adding 10 parts urea-formaldehydecondensation product which had become insoluble in water by reactingurea with formaldehyde in a molar ratio of 1:2 and then somewhatadvancing the polycondensation.

The obtained flexible polyurethane foam had a self-extingushing property(burning extent: 9.9 cm, burning time: 54 seconds). The smoke generationcoefficient C_(Smax) was 0.76 m⁻¹ and was lower than the value (1.03m⁻¹) of the ordinarily used polyurethane foam. Thus the polyurethanefoam in this example was very high in smoke resistance.

COMPARATIVE EXAMPLE 1

Using the same starting materials and operations as described in Example1, foaming was effected except for the use of 10 parts of urea insteadof N,N'-dimethylol urea condensation product in Example 1. The obtainedflexible polyurethane foam had a density of 25.3 kg/m³ and wasinflammable in the burning test of ASTM D-1692-59T.

EXAMPLE 4

The polyurethane foam obtained in Example 1, which had not yet beenafter-cured, was cut to a thickness of 2 cm and put in a mold having adepth of 2 mm, which had previously been heated at 100° C.andcompression-molded by applying a pressure of 100 kg/cm² by means of anelectric heating press set at 100° C. for 10 minutes, to obtain a foamsheet having a thickness of 2 mm. This foam sheet had a tensile strengthof 11.6 kg/cm² and an elongation of 115%.

COMPARATIVE EXAMPLE 2

Foaming was effected using the same composition as described in Example1 except that N,N'-dimethylol urea was not used.

The foam obtained in a cream time of 18 seconds and a rise time of 92seconds had a density of 28.9 kg/m³. This foam was cut to a thickness of2 cm and when this cut foam was compression-molded at 100° C. for 10minutes in the same manner as described Example 4 and taken out from themold, foam returned to the original foam shape and the compressionmolding was not possible.

FIG. 1 shows the foams prior to the compression-molding of the abovedescribed Example 4 and Comparative Example 2, which were cut to athickness of 2 cm. The left side and the right side in FIG. 1 show thefoams in Example 4 and Comparative Example 2 respectively.

These foams were put in the molds having depth of 2 mm and heated at100° C. as described above and compression-molded under heating under apressure of 100 kg/cm² for 10 minutes. The obtained results are shown inFIG. 2. The foam in Example 4 was molded into a sheet having a thicknessof 2 mm and the shape was maintained (left side foam), while when thefoam in Comparative Example 2 was taken out from (right side foam).

As seen from these photographs, it has been found that the flexiblepolyurethane foam according to the present invention has the heatcompression-molding ability which has never been possessed by theconventional foam.

EXAMPLE 5

The foaming was effected in the same process and operation as describedin Example 1 except that 10 parts of N,N'-dimethoxymethyl urea (made byMitusi Toatsu Chemicals Inc. trademark: Uramine T-101) instead ofN,N'-dimethylol urea and 3 parts of diammonium hydrogenphosphate as alatent acid catalyst were used. The polyurethane foam obtained in thecream time of 25 seconds and the rise time of 180 seconds had a densityof 26.7 kg/m³. This foam was cut into 2 cm and the cut foam was moldedat 100° C. for 10 minutes in the same manner as described in Example 4to obtain a foam sheet having a thickness of 2.0 mm. This foam sheet hada tensile strength of 6.96 kg/cm² and an elongation of 230%.Furthermore, the obtained polyurethane foam sheet had theself-extinguishing property in the burning test of ASTM D-1692-59T.

EXAMPLE 6

The foaming was effected in the same process and operation as describedin Example 1 except that 10 parts of commercially availableurea-formaldehyde condensation product (made by DAIWA Chemical Co., Ltd.trade mark: UMT-G) was added as the urea derivative. The foam obtainedin the cream time of 20 seconds and the rise time of 129 seconds had adensity of 23.5 kg/m³. This foam was cut into a thickness of 2 cm andthe cut foam was molded at 100° C. for 10 minutes in the same manner asdescribed in Example 4 to obtain a foam sheet having a thickness of 2.0mm. This foam sheet had a tensile strength of 5.17 kg/cm² and anelongation of 135%.

The obtained polyurethane foam sheet had the self-extinguishing propertyin the burning test of ASTM D-1692-59T.

EXAMPLE 7

The foaming was effected in the same process and operation as describedin Example 1 except that 10 parts of N,N-di(β-hydroxyethyl) urea wasused instead of N,N'-dimethylol urea. The polyurethane foam obtained inthe cream time of 25 seconds and the rise time of 180 seconds had adensity of 27.5 kg/m³. This foam was cut into a thickness of 2 cm andthe cut foam was molded at 100° C. for 10 minutes in the same manner asdescribed in Example 4 to obtain a foam sheet having a thickness of 2.0mm. This foam sheet had a tensile strength of 6.31 kg/cm² and anelongation of 125%.

The obtained polyurethane foam sheet had the self-extinguishing propertyin the burning test of ASTM D-1692-59T.

EXAMPLE 8

The foaming was effected in the same process and operation as describedin Example 1 except that 10 parts of N,N'-dimethyl urea was addedinstead of N,N'-dimethylol urea. The foam obtained in the cream time of17 seconds and the rise time of 103 seconds had a density of 24.1 kg/m³.This foam was cut into a thickness of 2 cm and the cut foam was moldedat 100° C. for 10 minutes in the same manner as described in Example 4to obtain a foam sheet having a thickness of 2.0 mm. This foam sheet hada tensile strength of 2.17 kg/cm² and an elongation of 160%.

The obtained polyurethane foam sheet had the self-extinguishing propertyin the burning test of ASTM D-1692-59T.

What is claimed is:
 1. A method of producing a flame- andsmoke-resistant flexible polyurethane foam sheet which comprises (1)reacting a system consisting essentially of a polyhydroxyl compound anda polyisocyanate with a urea derivative selected from the groupconsisting ofa. a compound represented by the general formula ##STR5##wherein at least one of R₁, R₂, R₃, and R₄ represents a member selectedfrom the class consisting of a hydroxyalkyl group, an alkoxyalkyl group,an alkyl group and an aryl group and wherein any of R₁, R₂, R₃ and R₄which does not represent a member from said class represents a hydrogenatom; and b. a urea-formaldehyde condensation product prepared byreacting 1 mole of urea with 1 to 4 moles of formaldehyde in thepresence of an alkali catalyst, said reacting with a. or b. being in thepresence of a catalyst for the production of the polyurethane foam, asurfactant and a blowing agent, wherein the polyisocyanate is used in anamount corresponding to an isocyanate index of 80-130 and the ureaderivative is used in an amount of 2-5 parts by weight based on 100parts by weight of the polyhydroxyl compound; and c. compressing thepolyurethane foam obtained in step (1) for 2-10 minutes at a temperatureof 80°-120° C. and a pressure of 5-200 kg/cm² to obtain said flexiblepolyurethane foam sheet.
 2. The method of claim 1, wherein thepolyhydroxyl compound has a number average molecular weight of 3,000 to6,000 and contains at most 70 mole % of primary hydroxyl groups in itsterminal group.
 3. The process of claim 1, wherein the polyhydroxylcompound has a number average molecular weight of 3,000 to 6,000 andcontains at most 70 mole % of primary hydroxyl groups at its terminals.4. The process as claimed in claim 3, wherein the polyhydroxyl compoundis selected from the group consisting of a polyether polyol and apolyester polyol.
 5. The process as claimed in claim 4, wherein thepolyhydroxyl compound is a polyether polyol.
 6. The process as claimedin claim 5, wherein the polyether polyol in poly(oxypropylene) triol. 7.The process as claimed in claim 1, wherein the polyisocyanate isselected from the group consisting of tolylene diisocyanate,diphenylmethane diisocyanate, diphenyl diisocyanate,chlorophenyl-2,4-diisocyanate, p-phenylene diisocyanate, xylylenediisocyanate and polymethylene-polyphenyl isocyanate.
 8. The process asclaimed in claim 1, wherein the polyisocyanate is tolylene diisocyanate.9. The process as claimed in claim 1, wherein the polyisocyanate usedhas an isocyanate index of 80-130.
 10. The process as claimed in claim1, wherein the urea derivative is a hydroxyalkyl substituted ureacompound.
 11. The process as claimed in claim 10, wherein thehydroxyalkyl substituted urea compound is selected from the groupconsisting of N-methylol urea, N,N'-dimethylol urea, N,N,N'-trimethylolurea, N,N,N',N'-tetramethylol urea, N'N'-diethylol urea andN,N-di(β-hydroxyethyl) urea.
 12. The process as claimed in claim 11,wherein the hydroxyalkyl substituted urea compound is N,N'-dimethylolurea.
 13. The process as claimed in claim 1, wherein the urea derivativeis an alkoxyalkyl substituted urea compound.
 14. The process as claimedin claim 13, wherein the alkoxyalkyl substituted urea compound isselected from the group consisting of N,N'-dimethoxymethyl urea,N,N'-dimethoxyethyl urea, N,N'-diethoxymethyl urea, andN,N'-diethoxyethyl urea.
 15. The process as claimed in claim 14, whereinthe alkoxyalkyl substituted urea compound is N,N'-dimethoxymethyl urea.16. The process as claimed in claim 1, wherein the urea derivative is analkyl substituted urea compound.
 17. The process as claimed in claim 16,wherein the alkyl substituted urea compound is selected from the groupconsisting of N-methyl urea, N,N'-dimethyl urea, N-ethyl urea andN,N'-diethyl urea.
 18. The process as claimed in claim 17, wherein thealkyl substituted urea compound is N,N'-dimethyl urea.
 19. The processas claimed in claim 1, wherein the urea derivative is an arylsubstituted urea compound.
 20. The process as claimed in claim 19,wherein the aryl substituted urea compound is selected from the groupconsisting of N-phenyl urea and N,N'-diphenyl urea.
 21. The process asclaimed in claim 1, wherein the amount of the urea derivative is 5-20parts by weight per 100 parts by weight of the polyhydroxyl compound.